This application claims the benefit of United Kingdom Application No. 0029120.3 filed Nov. 29, 2000.
This invention relates, in general, to a mechanism and apparatus for coherently recovering and interpreting data from an optical disc, such as a digital versatile disc read only memory (DVD-ROM) and more particularly, to an apparatus and/or method that operates to suspend data recovery and circuit operation in response to identification of regions of maximum reflectivity in the optical media that adversely affect recovery of modulated data.
With respect to the storage of data on optical storage media, such as on compact disc read only memory (CD-ROM) and DVD-RAM, a selected form of modulation encodes data into the surface of the media. In the context of DVD-ROM or DVD-RAM, an eight-fourteen modulation (EFM) scheme is used to encode binary data through the use of data xe2x80x9cpitsxe2x80x9d that are either magnetically or optically inscribed within, or manually embossed/stamped on the surface of the optical storage medium and undisturbed mirror regions. The length of the pit or complementary mirror is therefore indicative of the encoded binary information, subject to there being no defects associated with the formation of the pit or mirror.
The structure of data segments (or sectors) on an optical storage medium spiral outwardly from a center of the optical storage medium. The data segments are also indexed through the use of a header that is embossed (i.e., physically stamped) onto the surface of the optical storage medium, thus providing address and location information, such as track and sector numbers. The headers are individually indexed at the beginning of the disc for use in scanning. The headers have a precisely defined width dimension and are separated by a data sector of defined length.
From a perspective of data recovery, an on-track array of (typically) four photodiodes is used to recover the information stored on the medium. The four photodiodes provide an output current that varies according to an amount of reflectivity from the surface of the medium. More particularly, laser light is reflected from the marks and spaces with a data pit (i.e., a mark) providing an inferior reflectivity and hence a lower current than a space (that provides maximum reflectivity and hence maximum current). Essentially, with respect to data pits, destructive interference at the photodiode is generally indicative of the EFM modulation (in the exemplary instance of DVDs and CDs). The array is conventionally in the form of a 4-quadrant photodiode array in which a sum of the individual currents from the photodiodes is used to extract EFM data. More specifically, the four currents from the photodiode are converted to voltages in a pre-amplifier in a pick-up assembly before being presented to a read channel. The variation in the reflected laser light levels results in voltage signals of varying magnitude and duration (RF data), which are processed by the read channel to produce raw binary data.
When reading data from CD and DVD ROM discs, it is necessary to detect regions of the disc that contain defects. These regions take two forms: areas where the reflectivity of the disc is greatly reduced (defects), and areas where the data region is at maximum reflectivity (interrupts). Detection of the defective regions is necessary in order to perform certain holding and corrective functions in the player/recorder to maintain the data channel (principally the read channel) in a stable state during such defective regions. For example, at detection of a defect, data recovery, the phase lock loop and/or the tracking servo can be frozen. Failure to detect a defect otherwise results in the loss of coherent data in that particular region of defect. For example, once the device is on-track then the device should, ideally, be held on-track to avoid having to instigate a full recovery action for coherent data recovery. However, movement of the head pick-up will occur. During such cases, minimized disruption is desirable.
Furthermore, the ability to resolve mirror (i.e., high reflectivity) regions is important in the context of device operation since mirror regions inherently exist between adjacent tracks of an optical disc. Consequently, monitoring of the photodiode output during a track seek or jump mode where a read/write head (or pick-up assembly) moves radially across multiple contiguous tracks results in a sinusoidal modulation of the envelope of the RF data (known as the mirror signal or mirror modulation). Such mirror modulation is used to extract a mirror component that controls radial servo movement since the sinusoidal oscillation can be used to determine radial distance, location and relative movement between tracks by counting periods in the mirror signal. In contrast with read (RF) data on-track (which has a frequency of several megahertz), mirror modulation during track transition has a relatively low frequency of about 100 kilohertz. Defects, in general, can have an effect of corrupting a mirror modulation signal, with the defect causing de-focusing of the laser (at the photodiode) to the so-called xe2x80x9cdark levelxe2x80x9d, whereas interrupts generate spurious mirror signals. In other words, the defect takes the recovered signal components below a read frequency envelope/threshold associated with a data pit.
Methods for detecting low reflectivity defect regions are well documented and readily available in CD and DVD Read Channel integrated circuits. However, the issue of interrupts is not presently addressed in systems even though such interrupts potentially have the same detrimental effects on data recovery effects and device operation.
One aspect of the present invention concerns a method of determining interrupts in data on an optical disc, the data supported in a signal envelope subject to variation by mirror modulation and said interrupts. The method comprises the steps of (A) filtering the signal envelope to generate a first signal; (B) re-biasing the first signal to produce an intermediate signal having voltage swings attributable to the mirror modulation; (C) defining a slice level below a reference level to sample the mirror modulation to produce a mirror signal; (D) slicing the intermediate signal to generate the mirror signal containing a pulse resulting from a level transition through the slice level associated with re-biasing of the positive transition component; and (E) registering the presence of the pulse during the on-track mode of operation to identify the interrupt in data on the optical disc.
The objects, features and advantages of the present invention include providing a system that reliably detects both defects and interrupts in an optical disc media, and then operates to minimize damage to data recovery caused by such defects and interrupts.